Protective coating



NOV- 26, 1963 L. K. scHusTER ETAL 3,112,231

PROTECTIVE COATING Filed June 20. 1957 Clzrorrgium /Pe tcuzless Steelcwromz'zmz 75726 .Stainless Stee INVENTORS ATTORNEYS United StatesPatent O 3,1l2,23l PROTECTIVE CGA'HNG Ludwig K. Schuster, Philadelphia,and Alfonso L.

Baldi, Jr., Drexel Hill, Pa., assigners to Pennsalt CheniicalsCorporation, Philadelphia, Pa., a corporation of Pennsylvania Filed.lune 2t), 1957, Ser. No. 666,852 1 Claim. (Cl. 14S-6.2)

The present invention relates to protective coating of chromium typestainless iron alloys.

Although the above alloys are generally quite corrosion resistant, oneobject `of the present invention is to improve this resistance. Otherobjects include the provision of protectively coated alloys of theabofve type.

The above as well as still further objects of the present invention willbe more clearly understood from the following description of several ofits exempliiications, reference being made to the accompanying drawingsin which:

FIG. 1 is a side View of an automotive type Wheel hub cap;

FIG. 2 is a corresponding view of an automobile body trim strip; and

FIG. 3 is a perspective view of an external metal building panel.

In certain forms of automotive and building construction, it isdesirable to provide trim and other exposed surfaces in the form ofmetal that does not require painting and is generally free ofmaintenance problems. Very desirable metals for this purpose are thechromium type stainless iron alloys. These are ferrous alloys havingfrom about to 25% chromium content, little or no nickel, and may havecarbon, phosphorus, sulfur, silicon, molybdenum, aluminum, copper orother alloying ingredients in amountsv up to about 5% each. The lownickel content is a signilicant feature of these alloys since thisreduces their cost and avoids the supply problems associated with theuse of nickel in non-strategic constructions. Y

It has been discovered that the chromium type stainless iron alloys,although quite satisfactory from maintenance considerations, suierserious pitting in certain environments, e.g. in salt air or in contactwith building plaster, and this pitting is sharply reduced or entirelyprevented by a coating of an in situ formed combination `of hydratedmixed chromium oxides, the coating having from to 60% chromium byweight, about 40 to 95% by weight of the chromium being trivalent, theremainder being hexavalent, and the coating weighing from about 1 to 200milligrams per square foot of surface that it covers.

The coating `is readily made by applying to the metal surface an aqueoussolution essentially of chromic acid and a compatible reducing agent forthe chromic acid, and the coated surface is then heated to a temperatureof from 250 to 450 F. to cause the reducing agent to react with thechromic acid and leave a water-insoluble layer, the proportions ofreducing agent and chromic acid being such that from about 40 to 95% ofthe chromium in the chromic acid is reduced to trivalent form, and theiinal layer weighs at least about l milligram per square foot ofsurface. Coating weights of 1 to 10 milligrams per square foot 4arepreferred inasmuch as heavier coatings impart a colored appearance whichgenerally includes irregular optical interference patterns and maytherefore be objectionable for some purposes.

The properties of the coating can be varied by the type of reducingagent, its amount, and the reduction temperature. The reducing agent ispreferably one that does not leave Water-soluble salts in the iinalcoating layer. `Compatible reducing agents are those that when presentin the ICC above proportions do not cause any precipitation during theperiod in which the solution is kept as a solution, and are not drivenoff by the heat treatment, before they are oxidized. Organicpolyalcohols are very electifve reducing agents, and examples ofspecifically desirable polyalcohols include sucrose, invert sugar,dextrose, glycols, polyglycols such as diethylene glycol, glycerine,mannitol, sorbitol, triethanolamine and tartaric acid. Other reducingagents such as citric acid and hydroxylamine salts, such ashydroxylamine sulfate and even phosphorous acid and KCNS are alsosuitable. The reducing agent should not be so stable that it cannot beoxidized by the chromic acid, at least when it reaches the final bakingtemperature.

A coating bath containing both the chromic acid and the reducing agentshould not be permitted to stand long enough to form deposits ofinsoluble chromium compounds before the bath is applied to the surfaceand cured, that is, subjected to the nal heat treatment. To avoid this,the dwell of the reducing agent in contact with the chromic acidsolution should be of limited duration. The chromic acid and thereducing agent can even be applied as by spraying from a common sprayingjet, solutions of the separate ingredients being mixed together onlyjust before spraying.

The reducing agents listed above do not react very rapidly and can bestored in a chromic acid solution for at least `one day at roomtemperature F.) without deterioration.

In general, polyalcohols and their oxidation products are suitablereducing agents. These contain a carbon atom to which is linked analcohol group that is not protected against oxidation, and suchstructure appears to be eiective regardless of the remainder of themolecule, so long as the materials are not too volatile, that is, theyshould boil at temperatures Well above 212 F.

The amount of reduction depends upon the type and amount of reducingagent used and temperature of reduction. A general rule to follow is thehigher the reducing agent content and the higher the reductiontemperature the more reduction of the hexavalent chromium in the chromicacid. Coatings with 40 to 70% reduction of the hexafvalent chromium aremore highly colored, having a brown to brownish green appearance.Coatings having conversions of hexavalent chromium to trivalent chromiumfrom about 70 to 95% are extremely insoluble in water and in all typesof corrosive media likely t0 be encountered. Moreover, these highlyconverted coatings are almost colorless when in coating weights below 5milligrams per square foot.

The more highly converted `Coatings can be produced by selection of thecorrect amount of reducing agent as well `as the .proper conversiontemperature. A general rule is to use la lower CrOS-toareduci-ng agentratio, a higher reduction temperature, `and most effective, acombination of both. For coatings of highest conversion, it ispreferable .to incorporate the least amount of reducing agent and to usethe curing operation to effect maximum conversion. For example, toobtain a l milligram per square 4foot coating having 95% reduction, itis better to use, in the case of sucrose, ya ratio of 3 parts by weightof (lr03 to one part by weight of sucrose `cured at 388 F., than onepart by weight of CrO3 .to one part by weight of sucrose cured at 275 F.Curing temperatures above 450 F. produce coatings that give lessprotection `against pitting. A curing temperature of 325 to 390 F. givesbest result-s with 'coating weights of l to l0 milligrams per squarefoot. In the upper portion of this coating weig-ht range (above 5milligrams per square foot) somewhat higher ltemperatures ranging to 425F. are also extremely effective.

The percentage of trivalent chromiu-m in the coating can be determinedby dissolving it in a 2 to 40% aqueous solution of NaOH by weightat 160F. or higher, titrating the hexavalent content of the resulting solutionwith sodium thiosulfate in the presence of potassium iodide, andsubtracting this content from the total chromium content which isdetermined by oxidizing another dissolved coating sample with H2O2 andagain titrating with sodium thiosulfate in the same way.

In general the greater the triv-alent chromium content, the moreinsoluble the coating and the more resistant it becomes to chemicalattack. However, the greater' resistance to attack requires longertreatment for dissolution of the coating in the `above analyticalprocedure. The hexavalent chromium content appears to be reducedsomewhat `during the more severe dissolving operations because of thechemical action on the metal. Apparently the nascent hydrogen generatedby the reaction effects the reduction. The hexavalent chromium contentthat `any coating bath provides can also be determined by applying thecoating bath in exactly the same way to other metals, or Ito surfaceslike glass, which do not interfere with the analysis. Glass, even :ifattacked by alkali, does not appear to cause any change in thehexavalent chromium content. Coatings on glass can be dissolved in moreconcentrated alkali, such as those containing 40% NaOH by weight, andthe coating formed on glass appears to be identical to that formed onmetal.

The coating solution can be applied by immersion, ooding, spraying,roller coating, electrostatic spraying, etc. Roller coating, immersing,and flooding are particularly suitable for continuous coating of stripmetal. The thickness of the coating depends upon the concentration of`the solution, and if applied by roller coating, the roll pressure androll surface characteristics also affect the thickness of the finalcoating.

It has been found advantageous to incorporate in the coating solution asmall amount of wetting agent in the order of .005 to 0.3% by weight toimprove the wettability of the metal surface by the coating solution.With such a wetting agent in the treating solution, lthe surface of themetal is more uniformly and more rapidly wetted so that the entiretreatment time is held to a mini-mum. The final `coating is `also moreuniform and precleaning problems are minimized. Furthermore, oxidizablewetting agents such -as are formed by condensing 3 mols of ethyleneoxide with p-(n-octyl)phenol, may be use to take the place of some ofthe reducing agent. Longchained `alkyl sulfatos are suitable andtertiary butyl alcohol will be effective although it does not cause anyappreciable reduction and is generally required to be lin higherconcentration, e.g. 0.5%, to provide really effective wetting. lngeneral, however, anionic, cationic or nonionic wetting agents includingquarternary ammonium compounds of long-chain alkyl or aralkyla-mines,can be used.

In some cases, Where :metals are difficult to wet, the amount of wettingagent used may lead to foaming. This diliiculty may be minimized byadding to the bath an anti-foaming agent such as water-solubilized alkylphosphate type wetting agents. For example. a 0.001% addition ofmonolauryl phosphate completely esterified with octaethylene glycol, toa bath containing 0.006% of the above-mentioned `octyl phenylpolyethylene oxide alcohol wetting agent, gives very effective wettingwithout foaming difficulties.

The following examples indicate some of the ways in which the inventioncan be practiced.

Example I AiSl Type 430 stainless steel, bright automotive finish .02inch thick in `coil form, was processed through the following steps:

(a) Cathodically clean in an aqueous solution containing 16 `grams KOHper liter at 14C-160 F., at a current density of 40 amperes per squarefoot for 6 seconds.

(b) Tap water rinse.

(c) Clean anodically in an aqueous solution containing 4 16 grams KOHper liter at 140160 F., at a current density of 40 amperes per `squarefoot for 6 seconds.

(d) Tap water rinse.

(e) Pass strip between grooved rubber rolls ooded with an aqueoussolution containing 0.3% CrO3 and 0.1% cane sugar `at 75 F., therebyleaving a uniform film of the solution on the sheet.

(f) The filmed strip is then heated to a temperature of 390 F., bypassing it for 21/2 seconds between banks of ceramic gas burners heatedred hot by burning gas.

The resulting cured coating weighs 6 milligrams per square foot and asample of the coated metal exposed to a salt spray cabinet, 20% NaCl at95 F., shows no corrosion after l months exposure. Uncoated samples fromthe same stock `are badly covered with corrosion products after a 1weeks exposure to the same treatment.

Example Il Six groups of 3 inch by 5 inch AISI Type 430 stainless steelpanels, bright automotive stock, .025 inch thick, were cleaned bybrushing in an aqueous solution containing per `liter 25 grams Na2HlDO4,5 grams NaOH, and 5 grams of the wetting agent made by condensing 3 molsof ethylene oxide with p-(n-octyl)phenol, at 180 F. for 1 minute. Afterrinsing with water the cleaned panels are coated by dipping them in anaqueous solution containing Cr03 and enough cane sugar so that theweight ratio of CrOS/sugar is maintained at 3/1. Each group of panelswas coated with a different solution as specified below. The coatedpanels were then heated in an electric furnace for 9 seconds to attain atemperature of 350 F. on the surface. The following table shows thepercent of CrO3 used, the cured coating weight per square foot, theappearance of the coating, and the performance in salt spray exposure.

Percent Coating Performance in Salt OrO; Weight, Coating AppearanceSprny at F., 20%

mg./sf1. it. NaCl. 1 month Untrcatcd.. Very poor performance surfacecovered with rust. .1 1 Not noticeable Slight and localized corrosion..25 5 Almost; colorless Slight and localizcd mrrosion. 5 8 Very lightbrownish Very slight antllocalizcd tinge uniform. corrosion. 1.6 27Light blue uniform No corrosion.

coating. 2.8 5G Light purple coating.. Do. 6.0 96 Optical interferenceDo.

pattern. 11.0 212 do D0.

Example III Several 3 inch by 5 inch Type 430 stainless steel panels asin Example II were cleaned, coated and cured as dcscribed in Example H,except that in all cases 1.5% CrO3 acid was used with various types andamounts of reducing `agents as listed below:

Weight Ratio: Performance in Salt Reducing Agent Used CrO/Rcducing AgentSpray Exposure 20% NaCl, 95 F.. 1 month I-IBPO; 3/1 Practically nocorrosion. H3PO3 3/2 Do. HsPOs 1/1 D0. Tricthanolnminc 3/1 Do. Do..l1/l. D0. Glyccrol 3/1 Do. Do 1/1 Do. Ethylene GlycoL 3/1 Do. D0 1/1 D0.Polyethylene glycol 3/1 Do.

having an average molecular weight of i).

Do 1/1 Do. Citric Acid.. 3/1 Do. Do 1/1 Do. Cano Sugar. 3/1 Do. Do 2/1Do. Uncoatcd Badly covered will! rust.

Example IV Weight Ratio Percent of CrO3 to Peak Curing ConversionAppearance Triethanolamine Temp,Y F. to Trvalent 325 40 Brown. 325 55Ligh Brovm. 425 75 Very Light Brown. 425 91 Greenish Tint. 425 95Colorless.

A feature lof the present invention is that very little time is neededfor the application of the chromium-con taining coating. The solution ofchromic acid land reducing agent need only be in contact with the metalsurface long enough to wet it propenly, and the curing operation need beno longer than required to attain the `desired surface temperature. Evena .fraction of a second at the specified `curing temperature issuicient. Sometime may be required to reach that temperature dependingupon the rate at which heat is supplied, the thickness and mass ofmetal, and the manner in which heat is supplied. Any source of heat isapplicable. The selection depends upon the desired conversion speed,shape of article to be heated, gauge, etc. Suitable heating sourcesinclude infrared lamps, `cnt-type ceramic or tubular metal gas burnersheated to incandescence, ordinary gas burners, induction heating, hotair, `and the like.

Curing .temperatures below 250 F. do not provide the striking results ofthe invention. For example, if the solution of fchromic Iacid andreducing agent is merely permitted to dry at 212 F., the coating is notnearly as resistant to chemicals `and in most cases is even soluble inwater. The solubility or the coating cured at 212 F. may diminish:somewhat upon standing for several days, but it does not `approach thatof a Icoating cured pursuant to the invention.

By reason of the high speed with which the coat-ings of the inventioncan be applied, they are readily incorporated in conventional metalmanufacturing or metal coating production lines. Such lines operate atspeeds sometimes as high as 600 [feet per minute or higher, `and at suchspeeds even a one-minute treatment would require quite a few hundredii'lt of additional line length.

The chromium-containing coatings of the present invention are veryeffective in weights as low as l milligram per square foot of coatedsurface, or as high as 100 or even 200 milligrams per square loot.Coatings heavier than about 50 milligrams per square foot do not seem toadd too much to the protection that is afforded, and are not desirablewhen the coated metal is to be sharply deformed, as by a drawingoperation. Even the 50 milligrams per square foo-t coatings do notwithstand drawing operations too well unless the conversion percentageis 70 o-r lower.

Coatings that have been cured at tempenatures of up to 375 F. can havetheir weights determined very rapidly by merely dissolving such coatingtro-m a sample portion of the coated metal. A solution of sodiumhydroxide in waiter at 20C-212 F. for 11/2 minutes is `etlective forthis purpose. The panel is weighed both before and after the coatingremoval and 'the weight loss vgives the coating weight. -lf desired theycoating weight can also be determined by dissolving it yolf in theabove manner and adding to lthe resulting solution an excess of sodiumperoxide or hydrogen peroxide. The peroxide is permitted to react tocompletion after which fthe solution is diluted to a standard volume andits color checked against a standard by means of a colorimeter. So longas :the precursor coating solution has a substantially unchangedcomposition, the chromium content shown by the colorimeter will indicatethe amount of coating on the tested section. This relationship holds ifbefore the testing the coating is cured at a temperature at least ashigh as 250 l?. A curing below this temperature leaves a product withsomewhat higher moisture content, and a suitable correction would thenbe needed. At or above 250 F. the moisture content of the coatingappears to be substantially constant.

Instead of ycolorimetric-ally determining the chromium content of thesolution in which the coa-ting is dissolved, this content can beobtained by titrating the solution with sodium thiosulfate as referredto above.

The coatings when cured at temperatures of about 400 F. or higher areresistant to solution by sodium hydroxide, and sodium hydroxidesolutions ot 30 or even 40% strength may be needed. Such strongsolutions dissolve some of the metal, however, and a correction shouldthen be applied. The `correction can be obtained by filtering oli theiron hydroxide tormed in the solution, preferably after the peroxidetreatment and after the peroxidized solution is neutralized withsulfuric acid and then made slightly alkaline with ammonia. From theweight of the iron hydroxide filtered off, and the analysis of thesteel, the chromium dissolved from the steel is determined. It appearsthat the chromium and iron are dissolved substantially simultaneously bythe treatment so that the correction is capable of good accuracy bothfor the colorimetric chromium analysis as well as for the overallcoating weight determination. A similar correction can be used with theother steels such as chromium-free alloys, but there the dissolved irondoes not significantly interfere with the colorimetric chromiumdetermination and no iron separation or confection is needed except forthe overall coating weight determination.

For the purpose of checking the coating during a continuous coatingoperation, the coating weight can be more readily determined by loweringthe curing temperature over a small portion of the metal surface so thata sample of that localized surface can have its coating more readilydissolved off. Where the metal is in the form of elongated sheets coiledup, such a low curing temperature can be restricted to one or `both endsof the coil, for example. The remainder or" the metal can be cured atthe best curing temperature, which is generally up near 390 F.

The above analytical procedures are also suitable for determiningcoating weights on other metals including plain carbon steels.

The coatings or" the present invention need not be relied on to impartany characteristic color to the base metal. This metal can Ibe itselfcolored as by plating with a dilferent metal such as copper, or byapplying to its surface an adherent colored coating such as in situformed metal oxide or sulde. The following examples show how thecoatings of the present invention improve the corrosion resistances ofsuch colored products.

Example V Panels as in Example II were given the following treatment:

(1) Clean cathodically in an aqueous solution containing 5% by weightNa2CO3 at 160 F. for 30 seconds, employing a current density of amperesper square foot.

(2) Rinse with tap water.

(3) Dip in an aqueous solution containing 20% by .'weight H2504 `for 1/2minute at 150 F. to activate the `steel surface (remove passive film).

(4) Rinse with tap water.

(5) Electroplate about .00002 inch copper on the metal surface usingRochelle salt copper cyanide bath. The

Rochelle salt copper cyanide bath contains the following constituents:

3.5 ounces per gallon copper cyanide 4.6 ounces per gallon sodiumcyanide 4.0 ounces per gallon Rochelle salts (potassium sodium tartarateKNaC4H4O-4H2O) 4.0 ounces .per gallon sodium carbonate The conditions ofplating used were 3 amperes per square foot, 75 F., 7 minutes.

(6) Rinse in tap water and dry by air blast.

Some of the above treated panels after the final rinse and beforedrying, were coated with an aqueous solution containing 2.1% CrOa, .7%cane sugar and .005% of the above-mentioned octyl phenyl polyethyleneoxide alcohol wetting agent. The solution was rolled on the panels andthe applied iilm cured to attain a temperature of 425 F. on the steelsurface (9 seconds-1500 F. electric furnace).

Example VI Example VII lOther copper plated panels of Example V afterthe final rinse were subjected to a bluing treatment by immersing themin an aqueous solution of 75 grams per liter potassium chlorate 75 gramsper liter ammonium nitrate 2 grams per liter copper nitrate at 180 F.for 10 seconds, followed by `tap water rinsing and drying. Sorne ofthese `blued panels were also given the chromium oxide coating treatmentof Example V, except that 1/2% CrO3 and .17% sugar was used.

The following table gives the color and salt spray performance of thepanels made in Examples V, VI and VII.

Treatment Color Performance in Salt Spray NaCl, 90F. 24 hours Coppereolor Completely discolored and Copper plate of Example V. moderatelycovered with corrosion products.

No change in appearance.

Copper plate plus chro- Bronze color" mium oxide coating of Nocorrosion. Example V.

Sulded copper coating oi Black Badly rusted throughout.

Example VI.

Sultded copper coating do Slight local pit corrosion.

und chromium oxide coating oi Example VI.

Blued copper coating of Blue Badly discolorcd and pit Example VII.corrosion throughout. Blued copper coating and Blnish-yellow. Veryslight corrosion and chromium oxide coating changing color.

0I Example VII.

Various colors can also be imparted to the above stainless steels lbymeans of heat or by treatment in fused baths such as a mixture of equalweights of sodium nitrate and potassium nitrate iat 300 C. Colorsstarting with a beautiful gold at about 500 F. to a blue at 1000 F. andblack at higher temperatures, can be produced by mere heating in air.These oxide films, however, are quite easily corroded and in themselveslower the corrosion resistance of the stainless steel. Application ofthe chromium oxide coating on top of these oxides, however, markedlyincrease corrosion resistance, as in the following:

Example VIII AISI Type 420 stainless steel panels .O inch thick weresolvent cleaned with trichlorethylene and then subjected to a heatingoperation by placing in an electric furnace for 1/2 minute at 1500 F.The resulting color was gold. Some of these panels were dip coated witha 11% CrO3 or 1/2% CrO3 using a CrO3/sugar weight ratio of 3/1 andcuring them in a furnace to attain a. temperature of 390 F. on thesurface (9 seconds in a G F. source). The 11% CrO3 solution gave a 5milligram per square foot coating weight, and the 1/2% CrGs a 10milligram per square foot coating. The gold color was thereby madesomewhat deeper. Panels were subjected to a salt spray cabinet for 100hours using 20% NaCl at 90 F., with the following results:

Treatment Salt Spray Performance after 1U() hours Gold color hy heatingGold color by heating plus chromium oxide coating (5 milligrams persquare foot).

Gold color by heating plus chromium oxide coating (I0 milligrams persquare foot).

Badly corroded. a Almost no corrosion.

Almost no corrosion.

Example IX (l) Clean cathodically .010 inch thick panels of 17% chromiumsteel (0.40% carbon, no other significant ingredients) in aqueous 5% byweight Na2CO3, 160 F., 30 seconds, using a current density of 80 amperesper square foot.

(2) Rinse in tap water.

(3) Immerse for 5 minutes in an aqueous solution containing 10% byweight KMnO4 and 10% by weight NaOH at F.

(4) Rinse in tap Water.

(5) Dip in a solution containing 1% by Weight (lr03 and 0.3% by weightcane sugar, drain 30 seconds, and cure in a furnace to attain atemperature of 400 F. on the surface.

The resulting coating has an orange tint and offers excellent protectionto the stainless steel.

Another suitable technique for coloring stainless steels treated inaccordance with the present invention is by dipping in a strong aqueoussolution of sulfuric acid containing an etch inhibitor, as described inU.S. Patent 2,283,171, granted May 19, 1942. A still further differenttype of coloring for this purpose is described in U.S. Patent 2,618,578.

The coating baths containing chromic acid combined with reducing agentcan be stored for prolonged periods of time by adding to them a smallamount of a soluble maganese compound as described in U.S. Patent2,777,- 785. Some manganese compounds, such, as potassium permanganate,will cause after a short period of time the development of a slightprecipitate of manganese dioxide. This is not a sign of completedeterioration, however, since after another day or so the manganesedioxide precipitate will spontaneously dissolve. The presence of theprecipitate will interfere with the coating operation, and if the bathis to be used before the precipitate redissolves, it should be filtered.After spontaneous dissolution the bath can generally be stored forseveral weeks before a chromium-containing precipitate forms, toindicate that the bath is no longer suitable.

The coatings of the present invention can also be applied to fabricatedarticles such as forgings, machinings, castings and the like. Brushing,dipping or spraying can be used for the application of the precursorsolution and ovens or banks of heating lamps or gas burners for thecuring. Small objects such as rivets, nails, screws, etc., can be Wetwith the precursor solution and then cured by heated air whilecontinuously moved in a tumbling barrel. Rods, plates, strips, coils andstamped articles can also be heated in at least one of theabovedescribed ways.

The coatings of the present invention can also be changed in color bysubjecting them to a buling operation. For example, the coated metal ofExample Il, when buffed with a cloth butling wheel six inches indiameter and rotating at a speed of 3400 rpm., will take on a somewhatgold color. The coating color appears to change to gold under thefrictional treatment. A buiiing compound may or may not be used in suchan operation.

Another feature of the present invention is that the coatings behave asthough they are electrically conductive. One example of such behavior isthat electroplatings can be applied over the coatings. Chromium, forinstance, can be readily deposited at room temperature from an aqueoussolution f 400 grams of chromic acid and 4 grams H2804 per liter, thecoated metal being connected as a cathode, and using a cathode currentdensity of 50 amperes per square foot. The current density can rangefrom 20 to 100 ainperes per square foot, and the temperature can rangefrom about 60 to 140 F. The chromium deposits as a dull layer thatadheres to the oxide coating, but can be removed by electrolyticstripping, using the chromium plating as an anode in an aqueous solutionof 5% sodium hydroxide. After stripping, the mixed chromium oxidecoating of the present invention is found still adhering to the basemetal.

Copper, nickel, Zinc and the other metals can also be electroplated overthe coating of the present invention, in place of or in addition to thechromium, and any or all of these metals can be so plated on the mixedchromium oxide coating regardless of the base metal which carries thecoating.

Although as described in applicants prior applications, Serial No.592,552, filed June 20, 1956, now U.S. Patent No. 3,031,333, and SerialNo. 615,755, filed October 15, 1956, now U.S. Patent No. 2,861,906,corrodible ferrous metals coated in accordance with the presentinvention show improved corrosion resistance if before the coating theferrous metal is subjected to a grain boundary etch, this does not applywith the chromium-type stainless iron alloys.

lron alloys containing insuiiicient chromium to be called stainless doshow improved corrosion resistance when a grain boundary etch is used. Atypical alloy of this low chromium kind would be Type AISl'` 8620 steelhaving the following composition (in addition to iron):

Percent Carbon 0.18 to 0.23 Chromium 0.4 to 0.6 Nickel 0.4 to 0.9Manganese 0.7 to 0.9 Molybdenum 0.12 to 0.25

However, even with the above low chromium alloy the grain boundary etchcan be omitted where the mixed chromium oxide coating of the presentinvention is not intended to remain permanently in place. For example,the above low chromium steel shows considerable scaling when subjectedto a heat treatment as for the purpose of hardening or tempering.However, if the unetched metal is coated with the mixed chromium oxidecoating of the present invention, a heat treatment at 1700 to 1800 F.for 20 minutes will produce very much less and uniform scaling. Thescaling will be substantially completely eliminated if over the mixedchromium oxide there is placed a loose layer of clay about 1A; inchthick. instead of the clay, a layer of borax, sodium silicate or sandcan be used.

The greatly improved corrosion resistance of the present invention asdescribed above is obtained with all of the chromium-type stainlesssteels whether the chromium content be as low as 10% or as high as 25%.Speciiically included are AIS Types 403, 405, 406, 414, 416, 430]?, 431,442, 443 and 446 steel. Alloys that contain appreciable amounts ofnickel usually have more inherent corrosion resistance and thisresistance increases as the nickel content increases. However, astainless steel having as much as 20% chromium and 11% nickel, such asTypes AiSl 301 and 302 ordinarily used in building panels, will stillshow some improvement when the coating of the present invention isapplied thereover.

Coatings of the present invention are also made more protective byincorporating certain resins in the coating solution. For example, theaddition to the chromic acid reducing agent solution of thebutadiene-styrene resin described in Example 1 of U.S. Patent 2,772,254,granted November 27, 1956, dispersed in water to make a mixed bathhaving equal amounts of chromic acid and resin, gives a resin-chromiumoxide coating that is much more resistant to salt spray and has muchbetter adhesion than the resin-free chromium oxide coating, particularlywith overall coating weights of less than 50 milligrams per square foot.Other butadiene-styrene resins both thermosetting or thermoplastic willalso give the same improvement, as will polyvinyl chloride, polystyrene,and alkyd resins either unmodified or oil-modified as with tung oil.Other resins such as polyethylene seem to show only slight effectswhereas the above-listed resins will increase the salt-spray corrosionresistance by about 50% vor more.

ln general, it is preferred t0 hold the maximum curing temperature downto not over 400 F. to keep from degrading the resin. As much as resincan be used if desired, and the advantages of the resin are felt with aslittle as 25% of the resin. Best results are obtained, however, at 30 to60%, particularly with adhesion under impact. All of the above resinconcentrations are based on the weight of resin solids as compared withthe total weight of chromic acid and resin solids present in the coatingsolution.

The following example shows the corrosion reduction obtained by theresin-chromic acid coating:

Example X A pressure container drawn in one piece from SAE 1010 steeland having a wall thickness of 12 mils, is cleaned by solvent degreasingwith trichloroethylene and 1A minute dip in 5% aqueousl Na2HPO4, thensprayed over its interior with an aqueous solution having 1% CrO3, 1/3%sucrose and 1% (solids content) of the butadiene-styrene (30:70)copolymer made as in Example Il of U.S. Patent 2,683,698, granted July13, 1954. The container was then passed through a hot air furnace heldat 400 F. for 3 minutes, the metal surface reaching a temperature of 375F. Upon cooling, the container which on analysis showed a coating weightof 10 milligrams per square foot, was subjected to saturated salt sprayat F., and showed only slight corrosion after 50 hours.

The product of Example X, even though no preliminary nitric acid etch itused, is suitable for many commercial purposes such as pressure-typecontainers for solutions of self-lathering shaving cream as described inU.S. Patent 2,655,480, granted October 31, 1953. The presence of theresin not only improves the adhesion of paints, but it increases thecorrosion resistance, particularly by acids such as are present inanionic detergents and in resin latices. Similar results' are obtainedwith the above resin replaced by the alkyd resin emulsion of Example lin US. Patent 2,634,245, granted April 7, 1953, the polyvinyl chlorideemulsion of Example I in U.S. Patent 2,689,242, granted September 14,1954, or the polystyrene emulsion of Example I in U.S. Patent 2,635,086,granted April 14, 1953.

The above resin-containing coatings are Very useful for application tocontainers for latex paints as well as detergents, and can be so usedwith or without a covering paint or enamel layer. Without the etch andwithout the resin, the protection although poorer, is still effectivefor the shaving-cream container use. On other metals such as copper, andchromium type stainless iron alloys similar improvements are provided bythe mixed chromium oxides themselves. Copper is also very effectivelyprotected against sulfide blackening by the resin-free coating withoutany preliminary etch, as well as by the resin-containing coatings.

The resin-containing coating mixtures more readily wet metal surfaces,particularly where the surfaces have not yet been too scrupulouslycleaned. A fairly short cleaning operation is accordingly all that needbe used, and if the metal as received is free from gross contaminants,an extremely short dip in aqueous alkali such as trisodium phosphate isall that is needed. Also because of the physical characteristics of thecoating dispersions when they contain resin in addition to the chromicacid and reducing agent, they can be more readily used to build upheavier coatings as by spraying. The coatings thus formed whether heavyor light also show better adhesion for paints that are applied overthem.

The mixed chromium oxide coating of the present invention can also beprepared with a chromic acid solution that has an appreciable proportionof sucrose-containing resins. By way of example, the presence of thephenol-formaldehyde-aniline-sugar resins of U.S. Patent 2,666,037,granted January 12, 1954, or the phenolformaldehyde-sucrose resinsdescribed in Chemical and Engineering issues of March 4, 1957, page 7,and March 25, 1957, page 27, can be added to the chromic acid solutionso as to provide a solid resin content of from 10 to 50% of the totalsolids. The presence of such resins in dissolved form increases theamount of reducing agent apparently because the sugar proportions of theresin molecules can be oxidized by the CrO3. It is accordingly preferredwhen these resins are used, to diminish the amount of separate reducingagent. In general, the decrease in amount of separate reducing agent isabout one-half the amount by weight of sugar that is polymerized in theresin. For example, a coating solution containing 2% chromic acid and 1%of the resin of Example 1 of U.S. Patent 2,666,037, need only havesucrose separately added in an amount of 1/2% by weight to give thechromium conversion of 95%, corresponding to the conversion of a similarresin-free coating solution containing 2/3% sucrose when both solutionsare used with curing temperatures of 390 F.

A feature of the present invention, particularly as applied to theresin-free mixed chromium oxide layer on chromium-type stainless steelalloys, is that the protective effect of the coating is retained eventhough the coated surface is buifed intensively enough to apparentlyremove all the coating. In other words, a buing operation, as with anordinary felt wheel 10 inches in diameter without the help of any bufngcompound and with the wheel rotating at 3500 rpm., can remove the colorordinarily present when a mixed chromium oxide layer of about 10milligrams per square foot is applied. Although the color is removed,the increased corrosion resistance of the coated stock remainssubstantially unaffected. Similar buiing of coatings having lowercoating weights, even down to as low as 1 milligram per square foot,will likewise not detract from the corrosion 12 resistance even thoughthe buffing is severe enough to apparentiy completely remove thecoating.

Referring now to the drawings, FIG. 1 shows a hub cap formed from Type430 stainless steel that had been coated before `forming in accordancewith the process described in Example I. The coating is partly brokenaway inthe figure to more clearly show it.

FIG. 2 illustrates a trim strip made of a sheet of Type 442 stainlesssteel and coated as in Example III after it was formed into the strip.

FIG. 3 is a building panel made of Type 410 stainless steel coloredorange while in strip form according to the process of Example IX, andwith the lchromium oxide protective coating applied over the color. Thispanel is of the interlocking type that can be used both as external orinternal wall facing.

The present application is in part a continuation of applications SerialNo. 615,755, tiled October 15, 1956, now U.S, Patent No. 2,861,906, andSerial No. 587,388, filed May 25, 1956, now U.S. Patent No. 2,911,332.The latter application is in turn a continuation-in-part of applicationsSerial No. 277,286, filed March 18, l1952, now US. Patent 2,768,103,granted October 23, 1956; Serial No. 278,481, filed March 25, 1952, nowU.S. Patent 2,768,104, granted October 23, 1956; Serial No. 371,427,filed Iuly 3() ,1953, now U5. Patent 2,777,785, granted January 15,1957; Serial No, 433,698, led June 1, 1954, now U.S. Patent 2,773,623,granted December 11, 1956; and Serial No. 458,668, tiled September 27,1954, now abandoned.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. it is, therefore, to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than `as specifically described.

What is claimed is:

A method of reducing scale formation during the heat treatment offerrous metal objects that normally form cale during such treatment at1700 to 1800 F., said method being characterized by the steps ofapplying to the surface of the metal before the heat treatment an insitu formed coating of a combination of hydrated mixed chromium oxides,the coating having from 20 to 60% chromium by weight, about 40 to 95% ofthis chromium being trivalent and the coating weighing at least 1milligram per square foot of coverage, and `then carrying out the heattreatment.

References Cited in the file of this patent UNITED STATES PATENTS1,824,082 Hernandez Sept. 22, 1931 2,022,798 Bengston Dec. 3, 19352,315,564 Thompson et al Apr. 6, 1943 2,393,663 lThomas et al Ian. 29,194-6 2,757,977 Lyon Aug. 7, 1956 2,762,732 Somets Sept. 11, 19562,773,623 Schuster et al Dec. 11, 1956 2,777,785 Schuster et al. Ian.15, 1957 2,846,342 Curtin Aug. 5, 1958 2,858,244 Long et al Oct. 28,1958 FOREIGN PATENTS 558,792 Great Britain Ian. 21, 1944 860,306 GermanyDec. 18, 1952

